The ongoing development and spread of mobile communication and computer technology requires inherently reliable, high-performance and inexpensive batteries having a high specific storage capacity. Great efforts are therefore being made worldwide to develop power storage mediums that meet these requirements. The battery system based on lithium-ion technology is believed to have the best chances of success in this respect.
One of the most frequently used cathode materials in commercial lithium-ion batteries is currently lithium cobalt dioxide, hereinafter “lithium cobalt oxide” (LCO) for short, on account of its high storage capacity and its favorable electrochemical behavior. When Li-ion batteries are produced, said material is first mixed, in powdered form, with conductive carbon, such as graphite or soot, and a polymer binder, such as polyvinylidene fluoride, which contribute, inter alia, to the compensation of the change in volume when the active material is charged and discharged, and is rolled, in the form of a paste, onto a metal film that is used as a current collector. In a subsequent step, an organic, liquid (e.g. lithium hexafluorophosphate in ethylene and dimethyl carbonate) or polymer-based (e.g. lithium salts in polyethylene oxide) electrolyte, followed by an anode layer, is applied in a cathode-supported cell structure.
The drawbacks to a structure of this kind are both the lack of cycle stability, i.e. the slow loss in capacity (degradation) upon each charge and discharge process, and the unsatisfactory temperature stability, which may lead to the battery catching fire in the event of a technical defect or improper use, on account of the high proportion of organic materials.
One approach for overcoming these drawbacks would be to dispense entirely with carbon-based functional materials when these batteries are produced, as is envisaged by the concept of the solid-state lithium-ion battery, for example. In a much-promising variant of this type of battery, the cathode and the electrolyte consist of a ceramic solid which, combined with an anode made of lithium or a solid that holds lithium, for example elementary silicon, guarantees a high degree of operational safety and considerably improved cycle stability. One condition for the production of this kind of battery consists in process steps that allow both sufficient densification of the functional layers and an effective ion-conducting and possibly electron-conducting connection within the layers and beyond the layer boundaries.
The literature currently discloses very few methods that describe the production of carbon-free lithium cobalt oxide cathode layers for constructing lithium-ion batteries. For example, Ohta et al. (Journal of Power Sources, 238 (2013) 53-56) describe a method for depositing LCO/lithium borate cathode layers on a niobium-doped lithium lanthanum zirconate electrolyte.
However, for the basic research, evaporation and sputter methods, such as atomic layer deposition (ALD), ion beam layer deposition and physical or chemical vapor deposition (PVD, CVD), were described for applying pure LCO to various substrates. For example, Kumar et al. (Materials Chemistry and Physics 143 (2014) 536-544) used a radio frequency magnetron sputter process to grow submicron (d<1 μm) epitactic LCO films on textured Au/Ti/SiO2 substrates. Stockhoff et al. (Thin Solid Films 520 (2012) 3668-3674) describe, for example, a method in which 200 nm thick LCO films can be applied to silicon wafers by means of ion beam sputter deposition.
Spin-on methods for depositing LCO layers using coating solutions that were produced by means of a sol-gel process are also known in the literature. For example, Gunagfen et al. (Applied Surface Science 258 (2012) 7612-7616) describe a sol-gel method using a polyvinyl pyrrolidone chelating agent to thus deposit submicron (d<1 μm) LCO films on silicon wafers by means of a spin-on process.
According to the prior art, all-ceramic lithium cobalt oxide cathode layers without carbon-based additives for improving the electron and ion conductivity can thus currently only be deposited on current collectors having layer thicknesses in the submicron (d<1 μm) range by means of technically complex evaporation or sputter methods or by means of spin-on methods using sol-gel based coating solutions. However, the deposition of pure LCO layers is in principle limited to a few micrometers on account of the anisotropic ion conductivity of the lithium cobalt oxide as a result of its layer structure and the resultant decrease in the current density as the layer thickness increases, the LCO crystallites being randomly oriented in the cathode. In principle, epitactically grown, pure LCO layers having preferred orientation can be produced on current collectors by means of evaporation or sputter methods; however, on account of the low rates of deposition, the thus produced layer thicknesses are generally limited to the micrometer range.
However, considerably thicker cathode layers, which can only be achieved by simultaneously increasing the achievable current density by mixing in a solid electrolyte, are needed to achieve the required high storage capacities in lithium-ion batteries. It is not in principle possible to produce composite electrodes of this kind by means of evaporation and sputter methods.